1. Field of the Invention
This invention relates generally to combline filters for microwave and radio frequency signals and, more particularly, to a structure for suspending a ceramic resonator above a cavity.
2. Description of the Related Art
Coaxial combline filters are widely used in wireless communication systems. More specifically, these devices are often employed to reject unwanted frequencies. When implemented as a bandpass filter, users can tune a combline filter to select a desired range of frequencies, known as a passband, and discard signals from frequency ranges that are either higher or lower than the desired range. The filters are commonly known as combline filters because they consist of a series of parallel structures that resemble the hair-combing teeth in a comb.
A cavity resonator confines electromagnetic radiation within a solid structure, typically formed as a rectangular parallelepiped. Because this cavity acts as a waveguide, the pattern of electromagnetic waves is limited to those waves that can fit within the walls of the waveguide. This restricted mode of wave propagation, usually referred to as the transverse mode, can be analyzed in several categories, depending upon the direction of wave propagation.
Transverse Electric (TE) modes have no electric field in the direction of propagation. In contrast, Transverse Magnetic (TM) modes have no magnetic field in the direction of propagation. Transverse Electro-Magnetic (TEM) modes have neither electric nor magnetic fields in the direction of propagation. While TEM modes can exist in cables, TE and TM modes are present in bounded waveguides, such as cavity resonators. Although a TEM mode could theoretically exist in a waveguide with perfectly conducting walls, real cavity resonators have lossy walls so they cannot support any TEM mode signals.
When designing a cavity resonator, the TM mode is particularly useful. To define TM mode signals in a cavity resonator, the electric field propagates down the center of the guide. Due to the standing wave pattern, the electric and magnetic fields approach zero along the resonator's metallic walls. In order to focus the electric field and permit a user to tune it, a cavity is placed inside the hollow space defined inside the filter's walls.
If the central resonator in a combline filter is metallic, the filter's Quality factor, commonly called the Q-factor, will be poor. This measurement is proportional to the resonator's frequency divided by its conductance, so the unloaded Q-factor will be relatively low if the resonator is made of a conductive material such as metal. Thus, some conventional filters have replaced metal resonators with ceramic resonators having higher dielectric constants.
In such filters, a non-metallic rod of ceramic material in the center of guide allows more precise tuning of the signal frequencies without producing the conductive losses typical of metallic resonators. While the magnetic field flows around the circumference of the cylindrical rod, the discontinuity of permittivity at the resonator's surface allows a standing wave to be supported in its interior. Thus, the electric field will flow down the long axis of the cylindrical resonator.
Because such resonators are typically hollow, a tuning screw may be inserted into a hole in the ceramic, thereby permitting easy adjustment of the rod's resonant frequency. A user may gradually advance the tuning screw, carefully monitoring the resulting variation in the frequency. A specific depth of insertion will correlate to a predictable resonant frequency.
In a traditional TM mode dielectric combline filter, the dielectric in the filter's ceramic resonator must be electrically connected to the housing. This connection often requires the use of complex techniques. For example, a layer of copper, an electrically conductive metal, may be applied to the outside of the ceramic resonator. In these implementations, however, it may be difficult to make the structure stable because it will be vulnerable to mechanical shock. Moreover, ceramic and metallic materials may have different thermal expansion coefficients, so heating and cooling may weaken the strength of the ceramic-metal junction.
Because copper will oxidize if exposed to the air, a second metallic layer is often added to protect the copper. Often, the fabrication process involves adding a passivation layer of lead or tin above the copper layer. In addition to protecting the vulnerable copper layer, this metal is suitable for soldering the ceramic component body into a housing. After plating the ceramic resonator with these metallic layers, solder is applied to couple the plated resonator to the metallic housing. Unfortunately, both the plating and soldering steps involve the use of complex metallurgical techniques, which are expensive and time consuming.
Accordingly, there is a need for a resonator that avoids the use of multiple metal layers, thereby simplifying the device and the process required for its manufacture. Furthermore, there is a need for placing a resonator inside a cavity without directly connecting the resonator to the conductive walls of the cavity.